1. Field of the Invention
This invention relates to data reduction arrangements of the type which remove redundant or insignificant data from a data stream. More particularly, this invention relates to data reduction schemes which receive information in the form of a pulse code modulated (PCM) data stream at a first rate of input, and provide a PCM data stream at a second, slower rate of output.
2. Description of the Prior Art
The prior art has thrust at data reduction problems with several techniques in an effort to more economically transmit and process data. A first such data reduction technique is denominated "run length encoding" and involves the replacement of redundant units of information with data corresponding to the number of successive redundant events. There are at least two substantial drawbacks associated with run length encoding, not the least of which involves the fact that data which contains only small amounts of redundancy will not benefit from such encoding, and may result in expansion of the data contained in the original data stream. The second drawback relates to the fact that run length encoding is generally applicable to remove redundancies in the data, but not insignificant data. The prior art provides variations of run length encoding to help compensate for some of the above-mentioned shortcomings. For example, in a data stream comprised of zeroes and ones where it is desired to remove redundant ones, circuitry is provided to switch over to an encoding scheme whereby redundant zeroes are removed if they outnumber the ones. Alternatively, the system may switch over to a direct data output line, thereby bypassing the data reduction circuitry.
Another data compaction technique known in the art involves bit by bit comparison of new data with old data and the generation of signals responsive to the differences between the data words which have been removed and the prior data sequences. This type of compaction is especially suited for reducing long data words into shorter words which contain only the differential information. Such a data compaction technique is not directly usable in systems which process data words of invariant length without reconstruction of the long data words. An illustrative examples of a data system which utilizes long data words which may contain only small differences between successive words relates to video image data transmission wherein video frames are reconstructed using the differential data. The complexities associated with this type of data compaction scheme render it cost effective only in long data word applications.
It is therefore an object of this invention to improve data reduction systems which reduce the amounts of redundant and insignificant data in a PCM data stream.
It is a further object of this invention to provide a data reduction system which is advantageously adjusted to provide a substantially constant rate of output data as the input data rate varies.